The present invention relates to an extended solid bulk composition carbon nitride (C.sub.3 N.sub.4) and its synthetic method. More particularly, the present invention relates to a starting material for the synthesis of C.sub.3 N.sub.4, a synthetic method for manufacture of C.sub.3 N.sub.4, and a novel bulk composition material C.sub.3 N.sub.4 with at least partial hybridization of sp.sub.2.
The .beta.-C.sub.3 N.sub.4 form has a diamond like structure with sp.sup.3 hybridization. The .alpha.-C.sub.3 N.sub.4 also has the diamond like structure with the difference being that one crystallographic axis is doubled. These forms are extended solids that have the potential to be among the hardest known substances synthesized. This hardness coupled with a low mass density would yield extremely hard and light materials. Cubic carbon nitride would also provide excellent wear resistance and corrosion resistance to substrate materials.
Tremendous efforts have been made to synthesize cubic C.sub.3 N.sub.4 or (.alpha.-C.sub.3 N.sub.4 and .beta.-C.sub.3 N.sub.4). These previous attempts have included high-pressure thermal decomposition of C-H-N precursors, ion and vapor deposition of nitrogen ions and carbon vapor, plasma decomposition of methane and nitrogen, shock-wave compression of organic molecules and supporting experiments. Hard carbon-nitrogen materials containing 60% C and 40% N have been deposited by pulsed laser ablation of graphite in combination with an atomic nitrogen source. However, each of these methods have resulted in materials with bulk nitrogen contents much lower than the expected 57 at. % for C.sub.3 N.sub.4. These previous attempts at synthesizing cubic carbon nitride have resulted in either amorphous films with very low nitrogen content or purported findings of extremely small crystallites of the cubic carbon nitrides resident in an amorphous carbon matrix. The purported finding of small crystallites in amorphous carbon matrix are unsuitable for any useful exploitation of the unique properties of a bulk composition C.sub.3 N.sub.4.
U.S. Pat. No. 4,033,764 issued to Wunning on Jan. 18, 1977 discloses a method for the preparation of a surface layer of .epsilon.-carbon nitride on ferrous metal parts. The Wunning patent discloses that heating dissociated ammonia with a small amount of carbon monoxide and carbon dioxide in the presence of ferrous metal parts allows the condensation of a thin film of e-carbon nitride on the ferrous metal parts. The exact composition of the carbon nitride in this patent is unreported, but is expected by one versed in the art to have low nitrogen content. This is because the source of nitrogen is ammonia, and it is quite likely that much of the nitrogen in the surface layer of carbon nitride remains at least partially protonated.
U.S. Pat. No. 4,664,976 issued to Kimura et al. on May 12, 1987 reveals a method for the preparation of a thin film of carbon nitride. The Kimura et al. patent discloses that a thin film of carbon nitride with carbon:nitrogen atomic ratio of 4:1 to 3:2 is formed by use of a sputtering device in which a RF power is applied to a carbon target in an atmosphere of nitrogen gas while heating the substrate between 50.degree. C. to 250.degree. C. This nitrogen content is also considerably low compared to the ideal C:N ratio of 3:4.
Maya, et al., "Carbon-Nitrogen Pyrolyzates: Attempted Preparations of Carbon Nitride", J. Am. Ceram. Soc., 74(7), 1686 (1991) reveals the thermal decomposition of organic compounds containing relatively high nitrogen contents. These thermal decompositions occur at temperatures in excess of 700.degree. C. in a high pressure bomb, and resulted in the formation of powders with atomic ratio of 3:2. X-ray Photo spectroscopy indicates that the hybridization of the thin films is sp.sup.2. This too is severely deficient in nitrogen content. The ordinary person skilled in the art will recognize that this powder is a form of graphite with high nitrogen impurities.
U.S. Pat. No. 5,110,679 issued on May 5, 1992 to Haller et al. reveals a sputtering method for the formation of carbon nitride thin films in a nitrogen atmosphere onto a heated substrate. The Haller et al. patent discloses that sputtering of carbon in a nitrogen atmosphere onto a heated substrate results in the formation of a carbon nitride thin film with nominal atomic composition of 3:2. The Haller patent is the first to disclose purported C.sub.3 N.sub.4 crystallites in an amorphous carbon matrix. X ray diffraction of the amorphous carbon matrix suggests that the entire matrix may have the sp.sup.3 hybridization characteristic of .beta.-C.sub.3 N.sub.4.
Niu, et al. "Experimental Realization of Covalent Solid Carbon Nitride", Science, 334, 261 (1993), discusses a method for preparing carbon nitride thin films by pulsed laser ablation of a graphite target combined with a high-flux atomic nitrogen source. This thin film has an average carbon to nitrogen ratio of 3:2. This paper also reports the formation of crystallites of .beta.-C.sub.3 N.sub.4 with its characteristic sp.sup.3 hybridization.
Chubaci, et al., "Properties of Carbon Nitride Thin Films Prepared by Ion and Vapor Deposition", Nucl. Instrum. Methods Phys. Res., B80, 463 (1993) discusses nitrogen ion bombardment and carbon vapor deposition on substrates to form carbon nitride. These films also have very low nitrogen content with atomic ratios varying from 5:1 to 14:1.
Li et al., "Nano-Indention Studies of Ultrahigh Strength Carbon Nitride Thin Films", J. Appl. Phys., 74(1), 219 (1993) discusses the formation of carbon nitride by d.c. magnetron sputtering of a graphite target in a nitrogen ambient atmosphere onto a substrate. These thin films have low nitrogen content but include crystallites of .beta.-C.sub.3 N.sub.4 with its characteristic sp.sup.3 hybridization.
Each of these methods for the formation of the cubic carbon nitride either yield carbon matrices which have been characterized as having small discontinuous crystallites with X-ray diffraction patterns consistent with carbon nitride. However, none of these prior methods have demonstrated any ability to synthesize bulk modulus extended solids of carbon nitrides with a C:N atomic ratio of 3:4.
Accordingly, the inventors hereof have recognized a need to provide suitable precursors having stoichiometry consistent with the final stoichiometry of the extended bulk modulus carbon nitride. The term extended bulk modulus is intended to mean a stable solid C.sub.3 N.sub.4 having at least one dimension equal to about 30-40.ANG.. The precursors provide the compositional and structural framework in which each amino-1,3,5-triazine unit in the bulk composition is about 3-4.ANG.. Thus, in accordance with the present invention, an extended bulk modulus of carbon nitride consists of about ten bound triazine units in any one dimension. In order to form a stable solid, it is believed that repeating units of four triazine rings are required.
Additionally, a need has been recognized to develop a synthetic method employing the precursors for the synthesis of cubic carbon nitride. By selecting the starting material to have a stoichiometry comparable to a polymerization unit for the extended bulk carbon nitride, the synthesis of the extended bulk carbon nitride can be conducted under thermal decomposition conditions wherein the carbon nitride is the yielded end product with gaseous byproducts.
There is also a great need for a precursor and a synthetic method with which said starting material for cubic carbon nitride may be manufactured.